Nanoscale 3-D images shed light on Earth’s core

Photonics SpectraFeb 2011
PALO ALTO, Calif. – Physicists can study the Earth’s
inner history, thanks to a technique that provides a picture of minerals interacting
at ultrahigh temperatures and pressures.

The new method, high-pressure nanoscale x-ray computed tomography,
is being developed at SLAC National Accelerator Laboratory and has enabled Wendy
Mao, a mineral physicist at Stanford University, to look at the separation of Earth’s
mantle from its iron-rich core approximately 4.5 billion years ago.

Mao has obtained unprecedented 3-D detail of changes in the shape
and texture of molten iron and solid silicate minerals under the same intense pressures
and temperatures they would encounter deep underground. She presented the results
of the first experiments with the technique at the American Geophysical Union’s
annual meeting in December 2010.

Combining a diamond anvil cell, which compresses tiny samples
between the tips of two diamonds, with nanoscale x-ray CT has allowed SLAC researchers
to capture images of material at high pressure. At millions of times atmospheric
pressure, only diamond can exert the necessary pressure without breaking under the
force.

“It is pretty exciting, being able to measure the interactions
of iron and silicate materials at very high pressures and temperatures, which you
could not do before,” Mao said.

Physicists have been investigating the elements that make up Earth,
trying to determine how the mantle and the core separated from and squeezed past
each other. If the planet got hot enough to melt both elements, the difference in
density could have sent iron to the bottom and silicates to the top. If the planet
did not heat up enough for that, one theory holds that molten iron could have been
able to move along the boundaries between grains of the solid silicates. But previous
experimental work has shown that at low pressure, iron forms isolated spheres, Mao
said, and spheres could not percolate through solid silicate material.

She said the results of her experiments using the CT technique
suggest that, at high pressure, the silicate transforms into a different structure,
and the iron takes on a more elongated, plateletlike form, spreading out on the
surface of the silicate, where it connects and forms channels instead of isolated
spheres.